The present invention generally relates to optical amplification of coherent optical beams, and more particularly to a method for driving a laser amplifier such that the distortion of output optical signals is minimized.
In optical telecommunication systems, the amplification of coherent optical beams together with optical information modulated thereon is a fundamental as well as an essential process. For this purpose, various laser amplifiers have been developed so far. Particularly, semiconductor laser amplifiers that amplify the optical beam of a selected wavelength have been studied intensively in relation to optical filters for use in optical telecommunication networks that employ wavelength multiplexing of optical signals.
When using the laser diode for optical amplification, the laser diode is driven with a drive current controlled slightly below a threshold level of laser oscillation. In the laser diode biased as such, an input optical beam supplied to an active layer of the laser diode induces a stimulated emission as the optical beam travels therethrough, and a desired optical amplification is achieved. In optical amplification, the optical beam having a wavelength that matches the resonant wavelength of the laser diode is amplified selectively. Thereby, the laser amplifier acts as the optical filter with a passband wavelength coincident with the resonant wavelength of the laser diode.
In such a conventional optical amplifier/filter device, it should be noted that the passband wavelength tends to change with the amplification of the optical beam, as the increased intensity of the optical beam, caused by the optical amplification, inevitably invites a depletion of carriers in the active layer of the device due to the enhanced stimulated emission. It should be noted that the depletion of the carriers causes an increase in the refractive index in the active layer, and such a change of the refractive index in turn causes a change in the effective length of the resonator of the device. This phenomenon occurs in the laser amplifiers that use the cleaved end surfaces for the resonator as well as in the DFB laser amplifiers that use Bragg reflection for the optical resonance.
FIG. 1 shows the structure of a typical conventional laser amplifier.
Referring to FIG. 1, the device includes a semiconductor layered body that in turn includes a single crystal substrate 1 of n-type InP, and a clad layer 2 of n-type InP is grown epitaxially on the substrate 1. On the clad layer 2, an undoped active layer 3 of InGaAsP is grown epitaxially, and a clad layer 4 of p-type InGaAsP is grown further thereon. On the clad layer 4, a contact layer 5 of InP is grown epitaxially, and a p-type electrode 7 is provided on the upper major surface of the contact layer 5 as illustrated. Further, an n-type electrode 6 is provided on the lower major surface of the substrate 1. The layered body extends between an input end and an output end, and an input optical beam I.sub.1 is supplied into the active layer 3 via the foregoing input end.
In operation, the device of FIG. 1 is biased by a bias current I.sub.b that is injected into the active layer 3 via the electrodes 6 and 7, wherein, unlike the usual laser diode, the level of the bias current I.sub.b is set slightly below the threshold level of the laser oscillation. When the input optical beam I.sub.1 is supplied to the input end of the laser diode thus biased, the optical beam I.sub.1 induces a stimulated emission of photons in response to the passage of the wavefront of the optical beam I.sub.1 through the active layer 3. In other words, the optical beam I.sub.1 is amplified as it travels from the input end to the output end while maintaining the coherency, and outputted from the output end as a coherent output optical beam I.sub.2.
In the above-mentioned operation of the optical amplification, it should be noted that a depletion of carriers occurs in the active layer 3 along with the amplification of the optical beam. With the increased intensity of the optical radiation in the active layer 3, the recombination of the injected electrons and holes is accelerated due to the facilitated stimulated emission as already noted. Such a depletion of the carriers in turn induces a change in the refractive index of the active layer 3 in correspondence to the region where the depletion of the carriers occurs strongly, and such a change of the refractive index in turn causes a change of the effective resonant length of the resonator of the laser diode. Thus, the operational characteristic of the laser diode amplifier changes dynamically in response to the level of optical amplification occurring therein.
FIG. 2 shows the characteristic curve showing the relationship between the gain and the wavelength of the laser amplifier of FIG. 1 schematically, wherein the characteristic i.e., the gain versus wavelength relationship, curve designated as g.sub.1 represents the characteristic for an infinitesimal optical input I.sub.1 i.e., a threshold level of the optical power of the input optical beam below which the corresponding characteristic curve does not change substantially with further changes in the optical power of the input optical beam. As can be seen in FIG. 2, the curve g.sub.1 has a peak indicative of a maximum gain in correspondence to a characteristic wavelength .lambda..sub.1 that is determined by the geometrical configuration of the device of FIG. 1. When the intensity of the input optical beam I.sub.1 increases, the characteristic curve shifts from the curve g.sub.1 to another curve g.sub.2 that has a characteristic wavelength .lambda..sub.2 longer than the wavelength .lambda..sub.1. When the intensity of the input optical beam increases further, the operational characteristic shifts to a curve g.sub.3 having a characteristic wavelength .lambda..sub.3 that is longer than either of the wavelengths .lambda..sub.1 and .lambda..sub.2.
The relationship of FIG. 2 implies the possibility that the operational characteristic of the optical amplifier of FIG. 1 may change dynamically when the input optical beam I.sub.1 is given in the form of optical pulses as shown in FIG. 3(A).
Referring to FIG. 3(A), the curve designated as S.sub.1 represents the waveform corresponding to an input data train "101010 . . . ," wherein the curve designated as S.sub.2 represents the waveform corresponding to an input data train "010101 . . . " When the data "1", characterized by a positive optical pulse enters to the device of FIG. 1 as the input optical beam I.sub.1 with the wavelength .lambda..sub.1, it will be understood that optical amplification is achieved at the beginning in accordance with the characteristic curve g.sub.1. There, an efficient optical amplification is achieved at the wavelength .lambda..sub.1 in correspondence to the peak of the gain.
With the progress of the optical amplification, however, the operational characteristic changes from the curve g.sub.1 to the curve g.sub.2 and further to the curve g.sub.3. Thereby, the gain of the device starts to decrease. As a result, there may appear a distortion in the output optical pulse as shown in waveforms S.sub.3 and S.sub.4 of FIG. 3(B), wherein the waveform S.sub.3 represents the output optical beam I.sub.2 corresponding to the input waveform S.sub.1 and the waveform S.sub.4 represents the output optical beam I.sub.2 corresponding to the input waveform S.sub.2. In the waveform S.sub.3, it will be noted that there appears a dip in correspondence to the peak of the waveform S.sub.1. Similarly, a dip appears in the waveform S.sub.4 in correspondence to the peak of the waveform S.sub.2. When such a distortion appears, the chance of the erroneous recognition of the data may increase. This problem becomes particularly acute for the high speed transmission of the data with a pulse rate of 1 GHz or more.